
Unless your were to mout the scanner extremely high on your mast, there is probably going to be very little difference at all in those three locations with regard to the performance of the radar.
The height of the scanner plays into the range it will be able to see, but at the same time, height increases the minimum dection range or the radar, and also exaggerates any rolling or pitching motion of the vessel.
The equation to calculate the distance to the radar horizon is 1.22 times the square root of the height of the scanner. This is the straight line distance from the scanner to the horizon. Assuming the scanner is mounted 15 feet above the waterline, the radar horizon is at 4.7 miles. If we double the height (30’) of the scanner, the radar horizon is now at 6.7 miles. At 45 feet above the water, the radar horizon is now 8.1 miles. So you can see that the height between these three locations really does not make a very appreciable difference in the radar rannge.
When looking at the radar equation, also consider that we are only looking at 1/2 of the total factors involved. We must also consider the height of our target, and its radar horizon. the height of the target makes it stick up OVER the radar horizon, thus increasing the range we can see it at. For example, let’s assume our scanner is 15 feet above the water. The maximum detection range for another 15 foot high target is about 9.4 miles (the sum ouf the distance to our radar horizon PLUS the distance to his radar horizon.) Most large commercial vessels measure up at 80 feet high and taller. A ship 80 feet tall can be detected at a maximum range of about 16 miles.
You may be saying at this point “I thought this radar was rated for 24 or 48 miles. What gives?” The rated range for a given system is determined solely by the ability for the electronics to transmit a pulse to a maximum range, and detect a returning echo. The rated range does not account for heights, weather, horizon, etc.
More critical than the height of the scanner, is ensuring it has a level platform to transmit from, especially on a sailing vessel. Pretty much all light marine radar systems (including all Raymarine units) have a vertical beamwidth of 25 degrees. This means, if you could see the radar beam, you would see 12.5 degrees of beam directed downward toward the water, and 12.5 degrees directed upwards, with the center aimed at the horizon. If the boat is heeled in excess of 12.5 degrees to port or starboard, the lower portion of the radar beam will be directed ABOVE the horizon, essentially rendering the radar blind in that direction. Conversely, on the low side, all of the radar energy is directed into the water, resulting in increased sea return (clutter) on that side. From this, you can clearly see the advantage of a selfleveling radar mount.
What I would look at for your situation, is which location is going to provide the minimum physical interference with your sails and rigging, while allowing for a self leveling radar mount.
